[Dual Shade] Introduce a shade header that is closer to the UX mocks.

The main changes:
* On handheld form factor, the header is a small clock that performs a
  shared element transition with the lock screen small clock.
* On large screens there is no header present, as well as on narrow
  screens (i.e. handheld) screens when no notifications are present.
* Use the shade-specific status bar on narrow screens, ensure it isn't
  blurred, and maintain it in position as the shade expands (i.e. do not
  translate).

Fix: 377479267
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.dual_shade
Test: Verified manually by opening and closing the notifications shade.
Test: Added unit tests for the new view-model state `showHeader`.
Test: Existing unit tests still pass.
Change-Id: I3b0f733a067998c4ffb64a62becc5830e3470d58